The present invention relates to a system for setting up circuits for the transmission of data at a constant flow rate between a plurality of stations. It is more particularly applicable to the transmission of digital data between a plurality of stations.
The transmission of digital data has numerous well-known applications in such fields as information processing, the transfer of texts, sound (telephony) or pictures (digital video).
In this field, there are two types of transmission networks, which have different functional characteristics corresponding to contrasting requirement classes.
A first type of network is known in the art under the generic name "packet switching networks". In this first type of network, the data are grouped and transmitted in the form of relatively small packets or blocks, along paths which can vary in space and in time. This type of transmission characterizes exchanges between computer programs, which generally consist of brief messages, interrupted by variable delays.
A second type of network is known in the art under the generic name "circuit switching networks". In this second type of network, continuous connections with a constant flow rate are established between equipment. This type of connection characterizes systems where the flow must be guaranteed, e.g. digital telephones or image duplication systems.
Both types of networks exist or have been planned on the scale of large geographical regions, or on a planetary scale. Their overall interconnection constraints, as well as constraints linked with the long distances, lead to technical choices which will not be discussed in greater detail here, because they fall outside the preferred use range of the invention.
By their very nature, other networks have a limited range, e.g. for serving an apartment block, an industrial site, or a laboratory. In the case of such networks, the tendency is to use procedures specifically adapted to the limited distances between the use points. They are consequently frequently known under the generic name of "local networks".
The prior art discloses three categories of such local networks, each coinciding with certain specific uses. However, when confronted with certain communication needs and particularly when it is necessary to have very high data flow rates guaranteed over short periods, these known systems suffer from disadvantages, which are reduced or eliminated by the present invention, as will be shown hereinafter.
A first category of local networks is of the packet switching type. Generically, the networks of this category are based on the subdivision of a single transmission channel, or a limited number of such channels. In order to regulate this subdivision, use is also made of probabilistic processes, such as those known under the abbreviation CSMA-CD (Carrier Sense Multiple Access, Collision Detection), as well as deterministic processes, such as time division multiplexing, or the passage of a right to transmit by appropriate signals.
This first category of local networks has characteristics which are well adapted to the normal communications requirements between programs performed on several computers, so that several "logic connections" are set up in this way via a simplified infrastructure, with in certain cases auxiliary services in connection with the control of errors and/or flows. However, it is generally impossible to ensure between two connected stations, an effective data flow rate exceeding a minimum fraction of the flow of the transmission channels used. This is due to the interference, which is impossible to forecast, from other traffic with regards to the subdivision of the connection means. Moreover, the cost of the automatic management equipment for the channel subdivision process is high and increases with the transmission speed.
A second category of local networks is of the circuit switching type, most of them being oriented towards telephony, or are directly derived from methods linked with telephony. Generally such local networks comprise a central switching member, such as the private exchanges known under the abbreviation PABX, with the same number of direct point-to-point outgoing connections as there are stations served.
By definition, the local networks of the second category have optimized characteristics for the switching of circuits. However, their initial orientation towards telephony means that in mose cases there is a limitation in the data flow rate for each circuit. In the opposite case, the cumulative cost of providing the same number of high data flow rate for each circuit. In the opposite case, the cumulative cost of providing the same number of high data flow rate connections as there are stations becomes prohibitive, because the effective traffic is sporadic.
The prior art also discloses a third category of local networks, combining on a single split connection, or a small number of such connections, the advantages of data switching by packets and by circuits. For example, a train of cyclic signals is divided in time into a certain nubmer of "channels", whereof some are operated in the circuit mode and others by packets.
The systems of this third category are particularly advantageous if several simultaneous services such as telephony, data circuits, switching by packets, etc. are desired at each branch station. However, the management and supervision circuits are onerous for the transmission of data alone and it is not possible to simply allocate or reallocate more than part of a flow to a pair of given stations. Moreover, for most of these networks arranged in loop form, substantial transmission delays are systematically introduced, these delays being inherent in the register insertion mechanisms involved.
This generic examination of the prior art local network systems shows that there is a need for temporary communications with a high data flow rate.